In many medical situations it is desirable to quantitatively determine, or measure, the recirculation rate or the recirculation efficiency of a biological or medical fluid to increase the benefits of, or decrease the time required for, a therapeutic treatment, or for diagnostic purposes. For example, hemodialysis (herein "dialysis") is an inconvenient, expensive, and uncomfortable medical procedure. It is, therefore, widely recognized as desirable to minimize the amount of time required to complete the procedure and to achieve a desired level of treatment.
In dialysis, a joint is typically surgically created between a vein and an artery of a patient undergoing dialysis. The joint provides a blood access site where an inlet line toga dialysis apparatus and an outlet line from the dialysis apparatus are connected. The inlet line draws blood to be treated from the patient, while the outlet line returns treated blood to the patient.
This joint may be an arteriovenous fistula, which is a direct connection of one of the patient's veins to one of the patient's arteries. Alternatively the joint may be a synthetic or animal organ graft connecting the vein to the artery. As used herein, the term "fistula" refers to any surgically created or implanted joint between one of the patient's veins and one of the patient's arteries, however created.
In the fistula a portion of the treated blood returned to the patient by the outlet line may recirculate. Recirculating treated blood will co-mingle with untreated blood being withdrawn from the patient by the inlet line. This recirculation, and the resulting co-mingling of treated and untreated blood, is dependent, in part, on the rate at which blood is withdrawn from and returned to the patient. The relationship is typically a direct, but non-linear relationship. It can be readily appreciated that the dialysis apparatus will operate most effectively, and the desired level of treatment achieved in the shortest period of time, when the inlet line is drawing only untreated blood at the maximum flow rate capability of the dialysis apparatus consistent with patient safety. As a practical matter, however, as flow rate through the dialysis apparatus is increased, the proportion of recirculated treated blood in the blood being drawn through the inlet line is increased. In order to select the flow rate through the dialysis apparatus, it is desirable to know the proportion of recirculated treated blood in the blood being withdrawn from the patient by the inlet line. This proportion is referred to herein as the "recirculation ratio". The recirculation ratio can also be defined as the ratio between the flow of recirculated blood being withdrawn from the fistula to the flow of blood being returned to the fistula. Recirculation efficiency may then be defined by the relationship: EQU E=1-R
where
E=Recirculation efficiency PA1 R=Recirculation ratio
Alternatively, recirculation efficiency may be equivalently expressed as the ratio of blood flow being returned to the fistula, but not being recirculated, to the total blood flow being returned to the fistula. Knowing the recirculation efficiency, the dialysis apparatus operator can adjust the flow rate through the dialysis apparatus to minimize the time required to achieve the desired level of treatment.
In the prior art, quantitative determination of recirculation ratio or recirculation efficiency has typically required laboratory testing, such as blood urea nitrogen tests, which take considerable amounts of time and which require withdrawing blood from the patient, which is recognized as undesirable.
A method and apparatus for qualitatively detecting the presence or absence of recirculation in a fistula is described in "FAM 10 Fistula Flow Studies and their Interpretation" published by Gambro, Ltd. based on research performed in 1982. The Gambro method and apparatus injects a quantity of a fluid having an optical density less than the optical density of treated blood into the dialysis apparatus outlet line. A resulting change in the optical density of the blood being drawn through the dialysis apparatus inlet line is qualitatively detected as indicative of the presence of recirculation. The Gambro method and apparatus does not quantitatively determine or measure a recirculation ratio or recirculation efficiency.
Devices which qualitatively determine recirculation by thermal techniques are also known.
A quantitative measurement of the recirculation efficiency of a bodily or medical fluid is useful in other therapeutic and diagnostic procedures as well. For example, recirculation ratios and efficiencies are useful for determining cardiac output, intervascular recirculation, recirculation in non-surgically created access sites, and dialyzer performance from either the blood side or the dialysate side of the dialyzer, or both.
It is known that the electrical conductivity of a fluid in a closed non-metallic conduit can be measured without contact with the fluid by inducing an alternating electrical current in a conduit loop comprising a closed electrical path of known cross sectional area and length. The magnitude of the current thus induced is proportional to the conductivity of the fluid. The induced current magnitude may then be detected by inductive sensing to give a quantitative indication of fluid conductivity. A conductivity cell for measuring the conductivity of a fluid in a closed conduit without contact with the fluid is described in U.S. Pat. No. 4,740,755 entitled "Remote Conductivity Sensor Having Transformer Coupling In A Fluid Flow Path," issued Apr. 26, 1988 to Ogawa and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference.
It is against this background that the differential conductivity recirculation monitor of the present invention developed.